#include <stdint.h>
#include <cpuid.h>

#define PG_PRESENT          0x0000'0000'0000'0001
#define PG_READ_WRITE       0x0000'0000'0000'0002
#define PG_USER_SUPERVISE   0x0000'0000'0000'0004
#define PG_WRITE_THROUGH    0x0000'0000'0000'0008
#define PG_CACHE_DISABLE    0x0000'0000'0000'0010
#define PG_ACCESSED         0x0000'0000'0000'0020
#define PG_EXECUTE_DISABLE  0x8000'0000'0000'0000

#define F_PRESENT      0b1
#define F_WRITEABLE    0b10
#define F_UNPRIVILEGED 0b100
#define F_WRITETHROUGH 0b1000
#define F_CACHEDISABLE 0b10000
#define F_ACCESSED     0b100000
#define F_DIRTY        0b1000000
#define F_MEGABYTE     0b10000000
#define F_GLOBAL       0b100000000

// PAGE MAP LEVEL 4 ENTRY
struct pml4e {
	uint64_t flags              : 6;
	uint64_t _reserved          : 6;
	uint64_t address            : 40;
	uint64_t _reserved_2        : 11;
	uint64_t execute_disable    : 1;
};

// PAGE DIRECTORY POINTER TABLE ENTRY 
struct pdpte {
	uint64_t flags              : 6;
	uint64_t _reserved          : 1;
	uint64_t page_size          : 1;
	uint64_t _reserved_2        : 2;
	uint64_t address            : 40;
	uint64_t _reserved_3        : 11;
	uint64_t execute_disable    : 1;
};

// PAGE DIRECTORY ENTRY
struct pde {
	uint64_t flags              : 6;
	uint64_t _reserved          : 1;
	uint64_t page_size          : 1;
	uint64_t _reserved_2        : 2;
	uint64_t address            : 40;
	uint64_t _reserved_3        : 11;
	uint64_t execute_disable    : 1;
};

// PAGE TABLE ENTRY
struct pte {
	uint64_t flags              : 9;
	uint64_t _reserved          : 3;
	uint64_t address            : 40;
	uint64_t _reserved_2        : 7;
	uint64_t protection_key     : 4;
	uint64_t execute_disable    : 1;
};

struct pml4e pml4[512];
struct pdpte pdpt[512];
struct pde   pd[512];
struct pte   pt[512];

static int get_maxphysaddr() {
	uint32_t eax, ebx, ecx, edx;
	__cpuid(0x80000008, eax, ebx, ecx, edx);
	return eax & 0xFF;
}

// entry point for amd64
void* amd64_shim(void *boot_info) {
	for (int i = 0; i < 512; i++) {
		pml4[i].flags = 0;
	}
	for (int i = 0; i < 512; i++) {
		pdpt[i].flags = 0;
	}
	for (int i = 0; i < 512; i++) {
		pd[i].flags = 0;
	}
	pml4[0].flags = F_PRESENT | F_WRITEABLE;
	pml4[0].address = (uint64_t)&pdpt;
	pdpt[0].flags = F_PRESENT | F_WRITEABLE;
	pdpt[0].address = (uint64_t)&pd;
	pd[0].flags = F_PRESENT | F_WRITEABLE;
	pd[0].address = (uint64_t)&pt;
	for (int i = 0; i < 512; i++) {
		pt[i].flags = F_PRESENT | F_WRITEABLE;
		pt[i].address = i * 4096;
	}
}